Reliable point to multipoint communications, referred to as multicast or broadcast depending upon whether a node is transmitting to a subset of receivers or to the entire set of receivers, is an efficient method to use communication resources. Prior work on multicast has been focused on wire-line networks. An example is the RMTP (Reliable Multicast Transport Protocol) protocol, S. Paul, K. K. Sabnani, J. C. Lin, S. Bhattacharya, ‘Reliable Multicast Transport Protocol (RMTP)’, IEEE Journal on Selected Areas in Communications, Vol. 15 No. 3, designed for the Internet. Reliable multicast over a wireless medium is a relatively new area and results from multicast over wire-line networks cannot readily be employed mainly because the wireless medium is prone to much higher error rates and because mobility introduces complications from a networking point of view. Moreover, in multi-access channels, nodes operate in an ad-hoc manner forcing a methodical approach to arbitrate the usage of the channel for efficient multicast. Most of the prior work on reliable multicast over a wireless medium has focused on designing efficient schemes for (a) enabling a transmitting node to acquire a channel and (b) allowing the receiving nodes to express their intent to receive data from the transmitter. Examples of such methods include Batch Mode Multicast proposed in M-T. Sun, L. Huang, A. Arora, T-H. Lai, ‘Reliable MAC Layer Multicast in IEEE 802.11 Wireless Networks’, Proceedings of ICPP 2002 and Leader based policy in J. Kuri and S. K. Kasera, ‘Reliable Multicast in Multi-Access Wireless LANs’, Wireless Networks, Vol. 7, 2001, pp. 359-369. An efficient parity based multicast scheme based on Forward Error Correction (FEC) is proposed in J. Nonnenmacher, E. W. Biersack and D. Towsley, ‘Parity-Based Loss Recovery for Reliable Multicast Transmission’, IEEE Transactions on Networking, Vol. 7, No. 4, Aug. 1998.
Due to differences in radio channel conditions among different wireless nodes, the frame error rate between any two pairs of nodes is different. This is one of the key observations which has not been addressed in previous studies. For example, in the Leader based method, once you decide on who the leader is, the sender proceeds in a stop and go fashion to deliver the intended data. Fixing the leader in this fashion can result in severe penalties to other users. Similarly the batch mode multicast simply focuses on minimizing the number of contention cycles needed to acquire the wireless medium than on exploiting the differences in channel conditions. Although the parity based scheme introduces the idea of retransmitting parity packets instead of original data packets, it does not address the issue of exploiting the fact that different users have different radio channel conditions. Accordingly, there is a need for an improved multicast methodology which takes into account differing radio channel conditions.